1. Technical Field
The present invention relates to an electro-magnetic bandgap structure having a function of shielding in a specific frequency band.
2. Description of the Related Art
Recently, not only wire/wireless broadcasting and communication technologies but services related thereto are rapidly developing. As a unit of GHz is used as a clock frequency unit of a printed circuit board (PCB), Power Integrity (PI), Signal Integrity (SI) and Electro-Magnetic Interference (EMI), which are caused by various on-off chips such as a digital block loaded and located on a multi-layer PCB or Simultaneous Switching Noise (SSN) generated by an electronic device package, have become important issues in a PCB design.
One of the most general methods of solving the EMI problem and the PI/SI effect due to the Simultaneous Switching Noise (SSN) generated by a high-speed digital system is to connect a decoupling capacitor between a power layer and a ground layer. However, numerous decoupling capacitors are necessary for reducing the Simultaneous Switching Noise. As a result, PBC space occupation of the capacitors as well as production cost increase makes it difficult to freely dispose other various components. Moreover, numerous capacitors are not effective for reducing noise in a frequency band higher than 1 GHz, which is a problem in a recent high-speed digital system.
In order to find a new method of solving such a problem, i.e., the Simultaneous Switching Noise in 1 GHz band, research is now being devoted to an electro-magnetic bandgap (EBG) structure capable of selecting frequency.
Previously-studied electro-magnetic bandgap (EBG) structures generally include a Mushroom type (MT)-EBG and a Planar type (PT)-EBG.
The MT-EBG, for example, has a structure in which a plurality of mushroom shaped EBG cells (see reference numeral 130 of FIG. 1) are inserted between two conductive layers to function as both a power layer and a ground layer. The schematic structure of the MT-EBG is shown in FIG. 1. FIG. 1 shows only four EBG cells in all for the convenience of showing the drawing.
In FIG. 1, the MT-EBG 100 has a shape in which a conductive plate 131 is further formed between a first conductive layer 110 and a second conductive layer 120, and in which mushroom-shaped structures 130 connecting the first conductive layer 110 and the conductive plate 131 by means of a via 132 are repeatedly disposed. The first conductive layer 110 functions one of the power layer and the ground layer, and the second conductive layer 120 functions the other. Here, a first dielectric layer 115 is interposed between the first conductive layer 110 and the conductive plate 131. A second dielectric layer 125 is interposed between the conductive plate 131 and the second conductive layer 120.
A capacitance component, which is formed by the second conductive layer 120, the second dielectric layer 125 and the conductive plate 131, and an inductance component, which is formed by the via 132 which connects the conductive layer 110 with the conductive plate 131 by passing through the first dielectric layer 115, are L-C serially connected with each other between the first conductive layer 110 and the second conductive layer 120, so that such a MT-EBG 110 can performs a function as a kind of a band stop filter.
However, since at least three layers are required for implementing the MT-EBG 100, the structure has a maximum disadvantage in that the number of layers increases. Therefore, there occur problems of not only PCB manufacturing cost increase but Lead Time increase.
In the meantime, a PT-EBG has a structure in which a plurality of EBG cells (see reference numeral 220-1 of FIG. 2) having a specific pattern are repeatedly disposed over one whole conductive layer to function as a power layer or a ground layer. The schematic structure of the PT-EBG is shown in FIG. 2. FIG. 2 also shows only four EGB cells in all for the convenience of showing the drawing.
In FIG. 2, the PT-EBG 200 has a shape in which a plurality of conductive plates 221-1, 221-2, 221-3 and 221-4 located in a plane that is different from a plane on which any one conductive layer 210 is located are bridge-connected with each other through specific parts (the corner ends of conductive plates of FIG. 2) by use of branches 222-1, 222-2, 222-3 and 222-4 made of a conductive material.
In this case, the conductive plates 221-1, 221-2, 221-3 and 221-4 having wide areas form low impedance ranges. The conductive branches 222-1, 222-2, 222-3 and 222-4 having narrow areas form high impedance ranges. Accordingly, the PT-EBG performs a function as the band stop filter capable of shielding specific frequency band noise through a structure in which the low impedance range and the high impedance range are alternately repeated.
Unlike the MT-EBG structure, such a PT-EBG structure has an advantage in that an electro-magnetic bandgap structure can be implemented by means of only two layers. However, the PT-EBG structure has a problem that it is difficult to make the cell smaller because the PT-EBG structure forms the EBG structure by use of only two impedance components without various factors.